This invention relates to a method for molding near net-shape glass articles. Specifically, this method is directed towards the production of glass articles which retain the shape imparted to them by, and do not warp as a result of, the pressing operation.
In glass pressing, the rapid and controlled exchange of heat between the glass and the mold equipment is extremely important. It is particularly important that the temperatures of the mold equipment be kept within a range which is most advantageous for the pressing process. This temperature should be below the "sticking" temperature, i.e., that temperature at which the glass tends to adhere to the mold equipment. However, the temperature should not be allowed to drop below the optimum forming temperature range as this may cause optical defects or checks in the product. Therefore, mold heating, prior to actual glass pressing, is an extremely important step in modern glass production and as such, it is well known in the art.
However, problems do exist in the prior art pressing methods, even though the first step, heating the molding apparatus to within the mold's operating temperature range, is utilized. Once the mold apparatus is heated, the molten glass charge is delivered to a receiving molding surface, whereupon the molding surfaces of the receiving portion and an opposing portion are essentially brought into pressing contact with each other to produce the desired article. Since the glass charge is being pressed therebetween it thus prevents the molding surfaces from undergoing actual contact.
During this pressing of a glass article, the glass charge is first displaced to form the part, then heat is removed during the dwell or press time. If heat is removed symmetrically about the center plane during dwell, then at the end of dwell the part will cool and not deform. However, if heat is removed non-symmetrically, more from the bottom molding surface than from the top surface, for example, then the part, upon cooling, will experience forces which deform it causing warping.
The problem with this prior art pressing method is the excess contact, i.e., excess gob-in-mold (GIM), time which the glass charge experiences with the receiving molding surface as compared to the opposing molding surface. The receiving and opposing molding surfaces experience equal contact time during the essential pressing stage; however, the receiving molding surface also undergoes contact time during the period starting at molten glass delivery and ending when essential pressing contact occurs between the receiving and opposing molding surfaces. This excess contact time, experienced by the glass charge in the receiving molding surface, resulted in more heat being removed from the surface of the glass charge in contact with the receiving portion, than from the opposing glass surface, whose only heat removal occurs during the contact time of essential pressing. As a result of this non-symmetric heat removal, articles regardless of their thickness and shape tended to warp. Thus, in order to obtain the desired shape the glass article required extensive secondary mechanical finishing such as grinding and polishing.
It is the solution of this warping problem to which the present invention is directed; i.e., the objective of this invention is to provide for a method of "symmetric heat removal" pressing for use in production of glass articles. Stated in another way, this invention's particular objective is to provide for a method of press molding glass articles which, following pressing, nearly possess the desired final glass article shape; simply termed near net-shape press molding for glass.